The research proposed here is a continuation of studies aimed at understanding the way in which synapses between mammalian neurons are established and maintained. In particular, we want to know how synapses are made in correct numbers between appropriate types of nerve cells, and the mechanism of their maintenance throughout life. These aims will be pursued in the peripheral autonomic system of guinea-pigs, rats and rabbits, using electrophysiological techniques, electron microscopy, and a variety of histological methods. The autonomic system has been chosen because a) the ganglion cells are accessible, b) much of their function and structure is already known, c) connections in ganglia are likely to obey the same rules as synapses in other parts of the mammalian nervous system, and d) some of the rules that govern synaptic connectivity in autonomic ganglia have already been established. The research will be carried out along several related lines: 1. The nature and basis of the selective synaptic connections evident in mature sympathetic ganglia. 2. The development of synapses, particularly the role and basis of synapse elimination. 3. The role of ganglionic targets in determining the type of innervation ganglion cells receive. 4. The intrinsic properties of ganglion cells as determinants of the innervation they receive. 5. The regulation of neuronal dendrites. The significance of this work lies in understanding how neuronal synaptic connections are normally controlled.